Breast cancer is caused by an interaction between genetic, environmental and hormonal factors. Estrogens have long been suspected to play a prominent role in breast carcinogenesis, although the nature and timing of the potentially damaging exposure is uncertain. One speculation is that prenatal exposure may contribute, based on the paradigmatic experience of several million individuals exposed in utero, from the 1940s to the 1960s, to the potent synthetic estrogen diethylstilbestrol (DES). Consequences of exposure to this carcinogen on the reproductive tract are multiple, including malignancies, phenotypic abnormalities and infertility. DES mechanism of action remains unknown, but there is evidence that it causes a variety of cellular toxicities, including particular types of genotoxicity, i.e., abnormalites of chromosome number (aneuploidy) and microsatellite DNA instability. To better understand the effects of prenatal DES exposure, the NCI established the Continuation of Follow-Up of DES Exposed Cohort Study to follow 4000 exposed, and 2000 matched, unexposed, women. Little is known about the effects of prenatal DES on other estrogen sensitive tissues, including the breast. Yet the average age of women in the cohort is 43 yrs, and i.e., the age when breast cancer incidence begins to rise rapidly. We are fortunate to have access to data and pathologic specimens related to breast disease in this unique cohort. Based on the data available regarding DES genotoxicity, we hypothesize that breast tumors in the exposed women, in contrast to breast tumors in unexposed women, will be characterized by increases in the particular genotoxicities DES has been shown to cause: i.e. microsatellite instability and the molecular counterpart of aneuploidy, loss of heterozygosity (LOH). To investigate this hypothesis, we will use a powerful system we have developed for examining small quantities of microdissected archival tissue samples to investigate the breast carcinomas of women in the cohort study. A profile or pattern(s) of abnormalities detected uniquely in the DES-exposed women's cancers may reveal particular loci, pathways or mechanisms, important not only to their malignancies, but to all estrogen-associated breast tumors.